This invention relates to air separation in general, and in particular to a method of generating power including an air separation step.
It is known to be advantageous in certain circumstances to recover work from nitrogen produced in a cryogenic air separation plant. One such circumstance is when there is a large local demand for oxygen but no complementary demand for nitrogen. In some proposals for so recovering work, the nitrogen is compressed and then passed to a gas turbine comprising a compressor for compressing air, a combustion chamber which uses the air compressor to support combustion of a fuel and an expansion turbine which expands the combustion gases. To this end, the nitrogen may be passed directly into the expansion turbine or into a region upstream of the expansion turbine. The expansion turbine is arranged to perform external work by driving the air compressor and an alternator to enable electricity to be generated. By this means most if not all of the energy requirements of the air separation can be met. Examples of such methods are included in U.S. Pat. Nos. 2,520,862 and 3,771,495.
The fuel used in the gas turbine is normally one of high calorific value, i.e. above 10MJ/m.sup.3. In some industrial processes in which oxygen is used, a low calorific value gas is generated and it is desirable to make use of this gas.
It has also been proposed in our European patent application EP-A-402 045 to recover work from nitrogen by heat exchanging it at elevated pressure with a hot gas stream and then expanding the resulting warmed nitrogen with the performance of external work. Such proposals do not however involve the combustion of a low calorific value gas stream.